1. Field of the Invention
The invention relates to a process for preparing vinyl chloride-vinyl acetate copolymers in the form of their solid resins by the suspension polymerization process in aqueous medium, and to the use of the solid resins thus obtainable as binders in coating materials.
2. Description of the Related Art
It is known that vinyl chloride-vinyl acetate copolymers with a low fraction of carboxyl-functional monomer units are suitable binders for coating materials. When copolymers of this kind are prepared by the solution polymerization process the products obtained feature high purity and ready solubility in coating-material solvents such as aliphatic esters or ketones. Drawbacks of the solution polymerization process are, however, the low conversion and the large quantities of solvents which arise when the copolymer is isolated by precipitation and which have to be recovered, at cost and inconvenience.
At an early stage, therefore, there were attempts to avoid these drawbacks by polymerization in aqueous medium. A problem with this, however, is that the vinyl chloride-vinyl acetate copolymers thus obtainable are of relatively poor solubility in organic solvents and lead to hazing of the coating-material solution.
DE-B 1105177 describes a process in which vinyl chloride and vinyl acetate are polymerized in aqueous suspension, the complete vinyl acetate fraction being introduced as an initial charge and the vinyl chloride fraction being introduced as an initial charge in an amount such that there is 40 to 60% by weight of vinyl chloride in the monomer mixture. DE-A 2409800 discloses a process for polymerizing vinyl chloride and vinyl acetate in aqueous suspension where all of the constituents of the polymerization batch, but only 50% to 80% by weight of the vinyl chloride employed are introduced as an initial charge, and the remaining VC fraction is metered in continuously after the start of the polymerization. A drawback with this metering process is that the solubility in aliphatic esters is inadequate, implying a hazy solution and numerous undissolved flakes.
In the process of DE-A 1770901 the suspension polymerization is carried out in the presence of a water-insoluble solvent which is, however, a good solvent for the monomers. In the process of DE-A 2206593 the suspension polymerization of vinyl chloride and vinyl acetate is carried out in the presence of cellulose ether as protective colloid and in the presence of organic solvent. A drawback with these processes is the solvent fraction, which must be disposed of or recycled, at cost and inconvenience.
DD Patent 135620 polymerizes vinyl chloride and vinyl acetate in the presence of comonomers containing carboxyl groups or hydroxyl groups in aqueous suspension, and introduces polyvinyl chloride or polyvinyl acetate as powders or in the form of an aqueous dispersion as an initial charge. The products thus obtained exhibit hazing in aliphatic esters, owing to the insoluble polymer fraction.